Memory B cells differentiate via a specialized pathway in the lymphoid germinal centers (GC) which is different from, and independent of the antibody formation and which includes somatic mutation and selections of cells with high affinity for the antigen. The regulation of the GC/memory pathway, which represents the most important adaptive immune mechanism, is poorly understood. This proposal will test three novel ideas about this regulation. One, it is postulated that GC/memory pathway of B cell differentiation is driven by specialized helper T cell (TH) subsets. These TH will be isolated by cell sorting and cellular cloning and their properties and mechanisms of action will be studied in vivo upon adoptive transfer into B cell-reconstituted scid mice immunized with hapten- carrier conjugates. The development of germinal centers (GC) in the spleen will be monitored by immunohistochemical techniques and the affinities of hapten-specific serum antibodies will be determined. The somatic hypermutation activity in the GC B cells will be studied by the sequence analysis of the rearranged VDJ segments that encode the H chains of hapten-specific Ab. Second, it is proposed that the GC/memory response may selectively stimulated with complexes of antigen (Ag) with antibody (Ab). This hypothesis will be tested using well-defined Ab of known isotype and affinity and the mechanism of the Ag/Ab complex activity in conjunction with the help provided by specific TH subsets and clones will be studied. Thirdly, it is proposed to determine whether the memory B cells home to bone marrow which appears to be a major site of the anamnestic Ab response and whether the bone marrow environment contribute to an additional molecular and functional maturation of the anamnestic antibody repertoire. The proposed studies will elucidate important mechanisms of immunological memory, provide tools for manipulation of the adaptive immune response and suggest novel strategies for vaccination.